Since we often get school internships (12-16 years old students), I had the chance to teach them a couple of times. Not an easy task, but fun
What I quickly noticed is that at least half of them don‘t have fun with CAD,programming or image manipulation. Since I find these 3 activities very fun its sometimes frustrating that most children actually struggeling with this kind of work. But this is quite normal. Its challenging to keep motivation high. But if a child don‘t like it, accept it and don‘t force it. You will notice highly motivated children by doing bonus work and asking question. So it might be a good idea to think in little tasks with bonus objectives.
Another aspect is forcing yourself to keep it very simple. As a professional you will take many things for granted (it sounds obvious, but you steadily trap yourself in overcomplicating it)
I think its more important to finish something, then creating something complex. In terms of Rhino its actually fun to create some sort of abstract form ( a simple spaceship for instance), which then can be printed. Giving a physical output is very important, because it leads to some sort of trophy-like experience. Programming is even more challenging. You won‘t be able to teach something useful at all, unless they are really motivated. Grasshopper as an entrypoint is actually not bad.
If we talk about true programming, its extremely difficult. Games are usually too complex, many loose motivation before finishing anything. Unreal Engine/Unity are way to complex as a starting point.Maybe using „Processing“ for creating a Snake game is a more realistic choice. When I was 15 I used BlitzBasic for a simple 2d game. I made 2d airplane fighter game, and the most challenging, but fun part was programming an homing missile which detects its enemies. That was fun and it worked Could be done in Rhino as well.
Another option is to create a little console app, which does some sort pseudo hacking. Maybe using Python and CV2 for webcam activation and edge recognition or simply reading cookies of your browser Hacking is kind of a superpower, kids love that